serialization: use internal endian conversion functions

These replace our platform-specific mess in favor of c++20 endian detection
via std::endian and internal byteswap functions when necessary.

They no longer rely on autoconf detection.

src/i2p.cpp

@@ -392,7 +392,7 @@ Binary Session::MyDestination() const
     }
 
     memcpy(&cert_len, &m_private_key.at(CERT_LEN_POS), sizeof(cert_len));
-    cert_len = be16toh(cert_len);
+    cert_len = be16toh_internal(cert_len);
 
     const size_t dest_len = DEST_LEN_BASE + cert_len;